ability (WoW)
There are certain standard actions that every character is capable of, such as jumping or attacking. However, members of each class, race, and sometimes tradeskill have certain actions and effects that distinguish them.
Loosely speaking, an ability is anything that appears in a character's spellbook. This will include all their class skills, as well as certain other actions they can perform. Most abilities are activated abilities that can be mouse-dragged from the spellbook to an action bar and used from there. A select few are passive bonuses, such as the Human racial skill, Diplomacy, which simply increases reputation gains by 10%.
See Ability lists.